Many four wheel drive vehicles are equipped with a plurality of range or splitter gears. These range or splitter gears perform a uniform multiplication on all of the speed change gears in the transmission. In industrial or agricultural vehicles, such as tractors, each range gear defines a velocity range that starts at zero and ends at a maximum forward or reverse speed. In addition, four wheel drive engagement for such vehicles is often independent of range gear selection and may be engaged or disengaged at any time.
It is known to monitor operating parameters of a four wheel drive vehicle and automatically engage four wheel drive when it will enhance vehicle operation. Typical operating parameters include wheel slip and transmission overload.
When a tractor is working in hard conditions, e.g. plowing hard ground, there is a risk that the gearbox will be overloaded if the operator fails to respond adequately to the conditions, either through lack of experience or a disposition towards brutal treatment of his vehicle.